Conventional methods for repairing the glass-lined equipment referred to:
(a) method for installing the caps and the bolts respectively made of anti-corrosive tantalum metal in a metal substrate of the vessel, and sealing the spaces between a glass layer and the caps or the bolts with Teflon packing (U.S. Pat. No. 2,631,360); and
(b) method of applying an anti-corrosive adhesive of an organic resin, such as an epoxy resin, to the damaged portion and solidifying the adhesive.
However, method (a) has difficulty in applying the method to the damaged portion on an area having an intricate surface form or on a widely extended area, and the Teflon packing may cause liquid penetration due to the degradation thereof; and since the heat resistance and the solvent resistance of method (b) are inferior, the application subjects of this method would be restricted.
Another conventional method includes a so called sol-gel process wherein an alcohol and a water solution of an organic metal compound, such as a metal alkoxide which serves as a starting material, are heated for forming a glass by dehydration and condensation reactions. In this sol-gel process, the higher the temperature for heating, the more the dehydration and the condensation reactions are accelerated, to form a more closely bonded glass. Accordingly, the heating procedure has to be carried out at a higher temperature (e.g., 700.degree. C. to 800.degree. C.) for producing a high density glass having no voids.
However, in applying the sol-gel process for repairing a damaged portion of a glass layer of glass-lined equipment, heating temperatures are restricted up to about 300.degree. C. or about 350.degree. C. to avoid the occurrence of cracks on the undamaged glass layer surrounding the damaged portion due to the difference in thermal expansion between the underlying steel substrate and the glass layer. Therefore, said sol-gel repairing method would require a considerable length of repairing time, and because of the shrinkage during the glass forming reaction, an excess thickness of the repairing agent layer in one application would cause a crack or an exfoliation in the repairing glass layer. Further, the glass formed under such low temperature remains porous.
U.S. Pat. No. 5,053,251 discloses a method for forming the repairing glass layer comprising the steps of forming a repairing glass layer with a repairing agent containing filler materials, such as glass powder, for preventing the repairing glass layer from substantial shrinkage during the glass forming reaction; filling the voids occurring between the particles of the filler materials with a repairing agent not containing the filler materials; and repeating said glass forming and void filling procedures. In this method, the heating of the repairing agent is carried out, in a range from about 300.degree. C. to about 350.degree. C. with a local heating device provided with a detachable heater unit being surrounded by a flexible insulating cover, and having magnets for attaching the heating device to the damaged portion. The filler materials used therein enable the repairing method to avoid the shrinkage of said repairing glass layer during the glass forming process, to maintain the characteristics required for the repaired damaged portion, such as anti-corrosion, and to reduce the number of repeated coatings for forming a repairing layer having a satisfactory thickness (e.g., 1 mm or more).
Although the repairing glass layers formed according to the aforementioned method has passed the test conforming to JISR4201 (steel ball dropping test to observe the bonding strength of the glass layer to the iron substrate), the bonding strength thereof is 1/10 or less Of the bonding strength of the undamaged glass layer. Further, when the repaired glass-lined vessel is in actual use, surface smoothness of the repairing layer is inferior to that of the undamaged glass layer, and residual material in the vessel tends to adhere to the repairing glass surface. A jet of highly pressurized water used to remove the adhered residuals would exfoliate or damage the repairing glass layer by the physical impact thereof.
To improve the bonding strength of the repairing glass layer to a metal substrate is a primary purpose of the present invention.